I'm a beginner, and have just gotten a 6 wt rod.
Been practicing on grass (dry flies) and have been to a stream once.
My local shop put 4 wt. WF line on the rod, and I'm wondering,
am I losing anything by not having 6wt, line on it?
Is it easier or harder to cast with?
Thanks.

There are situations where AFTMA standards mean you can use fly line a wt. heavier or a wt, lighter, but under lining by two wts is never a good idea. Have 6wt, line put on and you will instantly feel the differnece.